I have the 870, and I like it. If I had to buy it all over again, the only difference is I would have gone with 2 barrels. I have the 28" vent rib for trap/skeet, but would like to get something in the 20" range with ghost ring sights for competition/HD.... but thats probably not very helpful to you
As far as the pump guns go, I've heard good things about the Benellis, Winchesters, Mossbergs, and Remingtons. I like the Remington 870, but mostly out of personal preferance, and when you hold it, it feels like a quality gun.
And i need to know if pistol grips are a benefit or just something cool looking?
I don't think very many people are going to recomend the pistol grip only shotguns, but if you mean a pistolgrip/buttstock like an AR15 or AK47, you could go either way. I find that a plain stock (like what Smitty posted) is more ergonomic, and I can still tuck the stock under my arm to free up my other hand to open a door. Really, Its up to you.
You don't need the high comb or anything fancy on the stock, but if you like it, go for it. Personally, I think the plain stock is pretty comfortable. I prefer the my guns to look more classic than tacticool, so that might be part of your consideration about the stock style. For me, I like the traditional monte carlo stock more than a pistolgrip/buttstock combo.
I like pump shotguns more than autoloaders, but if you prefer the auto, go for it. Probably the best input that I can give is that you really do want to buy the two seperate barrels. I have no idea what your storage setup is like, but trying to get my 870 with the 28" barrel into the gun locker is a PITA. Some places will sell you a combo gun, that comes with both barrels.
If you decide to go with an autoloader, I think you can pick up Remington 1100s for pretty cheap, so I'd start looking there.
Should I go Semi or Pump? I can get a Semi Saiga 12, all done up, for about 600.
The Saigas are pretty cool guns (I have one of thier rifles, love it) but for me, I like pump guns. I've never felt like having to manually cycle the action slowed me down, but I'm usually shooting skeet, so I'm pumping while swinging to another target. Your milage may very. Right now, I think the prices of the S12 are a bit high, and that looks like its probably not a bad deal, but if you wait 6 or 7 months, the prices will probably come down. From a sporting stand point, I think a more convential shotgun works pretty well, but if the S12 is what you've got your heart set on, then get it. For home defense, I think its a 50/50 as to wether or not a S12 is any better than an 1100 or 870. This is mostly my opinion, and since I never was that great a trap shooter, and have never been in a gunfight, my opinion isn't worth all that much.
Theres a thread running right now comparing a pump shotgun to an autoloader like an S12, and it might be a good idea to check it out.
Hope this helps, and sorry if I kind of rambled a bit.
Chris "the Kayak-Man" Johnson